Reference Chapter Number

Question

I am writing a book with multiple chapters (in different sections, if that is relevant). Chapters are numbered. I want a way to transform the chapter name into the chapter number, someplace else in my text. That is, I want to say "See Chapter 4, which covers magic in depth". The chapter that is responsible for covering magic in depth may change over the course of the editing process: I want the chapter number to change with it automatically. I don't want to refer to the name alone. I am using LaTeX's autonumbering for chapters. Is this possible?

\documentclass[12pt,a4paper]{book}
\usepackage[latin1]{inputenc}
\begin{document}
\chapter{Sample 1}
    This is covered in more detail in Chapter X.
\chapter{Sample 2}
\end{document}

Answer 1

LaTeX provides this cross-referencing scheme out-of-the-box. Place \label{<label>} after a counter that can be referenced (like a chapter, section, subsection, theorem, ...) and use it elsewhere as \ref{<label>}. This will only return the counter number and nothing else:

\documentclass[openany]{book}
\usepackage[paper=a6paper]{geometry}% Just for this example
\usepackage{xparse,nameref}
\NewDocumentCommand{\chapref}{s m}{Chapter~\ref{#2}\IfBooleanF{#1}{ \nameref{#2}}}
\begin{document}
\chapter{Sample 1}\label{ch:first}
This is covered in more detail in \chapref*{ch:second}. Also see~\ref{ch:second}.

\chapter{Sample 2}\label{ch:second}
This is covered in more detail in \chapref{ch:first}. Also see~\ref{ch:first}.
\end{document}

The above example sets the \labels after every \chapter. They're referenced using \ref{<label>}, which clearly just returns the chapter number. However, with the addition of nameref we can grab hold of the chapter title as well, referencing it as \nameref{<label>}. A combination of \ref and \nameref is incorporated in what I defined as \chapref, which translates to Chapter~\ref{<label>} \nameref{<label>}. The latter \nameref is conditional on whether you call \chapref with/without a star *.

The above example can be made fully-functional with hyperref, providing inter-document hyperlinking.

The option [openany] was just so the two chapters fit on subsequent page and is not needed in your document.

Answer 2

As an alternative to Werner's code, you could also use one of the packages which set this up for you automatically. fancyref and cleveref are two which spring to mind. For example:

\documentclass[openany]{book}
\usepackage[paper=a6paper]{geometry}% Just for this example
\usepackage{fancyref}
\begin{document}
\chapter{Sample 1}\label{chap:first}
This is covered in more detail in \fref{chap:second}. Also see~\ref{chap:second}. \Fref{chap:second} covers this in excruciating detail.

\chapter{Sample 2}\label{chap:second}
This is covered in more detail in \fref{chap:first}. Also see~\ref{chap:first}. \Fref{chap:first} covers this in more detail.
\end{document}

Note that chapter is added automatically if the chap prefix is used in the label, and that \Fref and \fref produce capitalised and non-capitalised output. (I believe that cleveref automatises things further but I've not used it myself.)